1. Field Of The Invention
This invention relates to a process for reducing the color value of dye wastewater. More particularly, the invention relates to a process for removing dye bodies from wastewater and for converting the dye bodies into less chromophoric forms
2. Description Of The Related Art
Wastewaters from pulp and paper process effluents and from textile dyeing process effluents presents substantial environmental problems because such wastewaters contain substantial concentrations of "color bodies". For example, textile mills can generate millions of gallons of dye wastewater every day. Such wastewater contain colloidal bodies and suspended solids containing color compounds. Color compounds or "color bodies" can include dyes and/or dye by-products. Wastewater streams, that contains these dyes and/or dye by-products, often do not meet regulatory standards for wastewater discharge even after they have undergone treatment by a conventional process.
Various processes have been utilized to remove the colored colloidal bodies and colored suspended solids (hereinafter collectively referred to as "color bodies") and to reduce their associated high biochemical oxygen demand. One wastewater treatment process involves adding substantial quantities of lime to the wastewater from which pulp fibers and ligneous matter have been separated. The lime flocculates the color bodies and, thereby, reduces the initial color of the wastewater by 80 to 90 percent and significantly reduces the biochemical oxygen demand. This process has a significant disadvantage in that it requires high capital and operating expenditures. In addition, such wastewater treatment processes do not recover the pulp or ligneous material collected by the process. Finally, the quantities of lime employed by this process cause significant problems in lime handling, sludge removal, pH, and environmental quality of the wastewater that is ultimately released from the process.
Other processes for removing color bodies include treating the wastewater from a paper and textile dyeing process with lime and ferrous sulfate. This treatment is effective for separating a portion of the color bodies from the wastewater. However, dilution of the treated wastewater or a change in its pH releases the dye back into solution.
Some wastewater treatment processes pass the wastewater through an ion exchange resin to adsorb the lignins and their derivatives. Ion exchange resins, that are used for these purposes, generally comprise significantly cross-linked hydrophilic acrylic copolymers. These cross-linked hydrophilic acrylic copolymers adsorb the organic components by means of Van der Waals' forces. One of the primary disadvantages of the ion exchange processes is that the resin removes free chloride ions as well as the color bodies. When the ion exchange resin is regenerated, the released chloride ions increase in concentration in the system to undesirable levels. Ion exchange processes are, also, expensive when compared to other processes.
The degree of color in wastewater can be measured in "color units" as determined by a test process established by the American Public Health Association. This testing method measures the color content of water test samples by comparing the test samples to American Public Health Association color standards. The process uses colorimetric platinum-cobalt standards for measuring the "color value" of water that contains natural and synthetic compounds. The standard American Public Health Association color test process measures the color of a sample by visually comparing the test sample with the platinum-cobalt standards. One unit of color is defined as the amount of color that is produced by 1 milligram per liter (mg/1) of platinum in the form of chloroplatinate ion.
The American Public Health Association color test uses 50 milliliter (ml) "tall form" Nessler tubes. A standard chloroplatinate solution is prepared by dissolving 1,246 grams of potassium chloroplatinate (K.sub.2 PtCl.sub.6), which is equivalent to 0.5 gram of metallic platinum, and 1 gram of crystalline cobaltous chloride (CoCl.sub.2.H.sub.2 O) in distilled water containing 100 milliliters of concentrated hydrochloric acid (HCl). The solution is diluted to 1000 milliliters with distilled water. This standard solution is equivalent to 500 color units.
The standards used in the comparison test, are prepared in increments of 5 color units from 5 to 70 color units by dilution of the certified standard with known amounts of distilled water. The apparent color of the test sample is observed by filling a matched Nessler tube to the 50 milliliter mark with the test sample of water and comparing the test sample with the incremental standards. The comparison is made by looking vertically or downwardly through the tubes toward a white or specular surface placed at such an angle to the tubes that light is reflected upward through the columns of liquid in the tubes. When the color value exceeds 70 color units, the sample is diluted in known proportions with distilled water until the color value is within the range of the incremental standards. The measured color value is multiplied by the dilution factor to obtain the correct color value.
Very slight amounts of turbidity can interfere with the color value determination and, therefore, the samples showing visible turbidity are, desirably, clarified by filtration or centrifugation prior to the comparison of the test sample with the standards. If turbidity has not been removed by centrifuging or filtering, the color is recorded as "apparent color." When the turbidity has been removed by centrifuging or by other means, the color is recorded as "true color."
U.S. Pat. No. 4,088,573 to Nakajima discloses a process of clarifying wastewater containing acid dyes. The process involves contacting the wastewater with a dicyandiamide formaldehyde resin and sequentially adding at least 300 parts per million (ppm) of aluminum sulfate.
U.S. Pat. No. 3,947,248 to J. B. Powers discloses the use of cationic polymers to flocculate organic pigments in waste streams from polyester textile yarn and fabric manufacturing operations.
U.S. Pat. No. 3,969,239 to I. Shinohara et al. discloses a macroamine polymer as a high molecular weight flocculating agent in clarifying, precipitating, and decolorizing water. The polymer is used as the only flocculating agent in the treatment of wastewater to provide clarifying, precipitating, and decolorizing functions.
U.S. Pat. No. 4,005,011 to C. D. Sweeney discloses a process for the removal of color bodies in effluents from the manufacture of dyestuffs. The process treats the wastewater with lime to remove heavy metals. The wastewater is then clarified by carbon adsorption and subjected to a secondary biological treatment.
U.S. Pat. No. 4,724,045 to C. S. Ackel discloses a process for the decolorization of alkaline paper waste pulp. This process contacts the alkaline paper waste pulp with a strong mineral acid.
U.S. Pat. No. 5,071,587 to C. A. Perman discloses the removal of turbidity, metal contaminants, and organic contaminants from wastewater. The removal of these contaminants is accomplished by treating the water with tablets containing bentonite clay, attapulgite clay, polymeric coagulant, biocide, zeolite, and activated charcoal.
The above noted processes for treating colored wastewater containing large amounts of dyes have not been entirely successful. Many of the known processes, are not able to reduce the color of many wastewaters to acceptable levels for discharge to a public sewage stream. Other processes require additional treatments to adjust the pH to an acceptable range for discharge or require additional recovery steps to remove various components from the wastewater.
The invention provides a simple and efficient process for removing dyes or "color bodies" from aqueous wastewaters that are produced by dyeing or dye manufacturing operations. The invented process treats dye wastewater effluents or streams to provide an acceptable color value as established by agencies for regulating the discharge of the wastewater into a public sewage system.